The invention relates to the field of X-ray technology, in particular to an X-ray tube comprising a tube housing, a rotary anode having a target disk rotatable around a rotation axis, a cathode for producing a beam of electrons for generating X-rays upon impingement of said beam of electrons upon an emitting surface of the rotary anode, shielding means for intercepting undesired secondary radiation originating from said emitting surface.
An X-ray tube of this type is known form 0,009,946 EP. The known X-ray tube is provided with shielding means which are substantially X-ray opaque said means being manufactured in the shape of envelope. In the known X-ray tube the envelope contains the rotary anode target disk and is provided with an entry window to admit the electron beam originating from the cathode and an exit window for emitting the useful X-ray beam. The X-ray beam is produced by impingement of said electron beam on the target disk of the rotary anode at the focal spot area and will be further referred to as secondary radiation. The focal spot area of the rotary anode constitutes a part of the emitting surface of the target disk of the rotary anode. The X-ray beam emanates from the emitting surface in substantially 2xcfx80 solid angle. Only that part of the X-ray beam, which is transmitted through the exit window, contributes to the useful X-ray radiation of the X-ray tube. The remaining part of the X-ray beam contributes to the undesired secondary radiation. It is known that part of the primary electrons undergo scattering on the target disk. These scattered electrons contribute to the undesired secondary radiation as well. In the known X-ray tube the undesired secondary radiation is intercepted by the envelope. A drawback of the known X-ray tube is that the shielding means constitute a tube housing at the same time, increasing the actual volume of the shielding means and thus the amount and weight of the used material. Further, the manufacturing of the material which is suitable for shielding purposes is expensive and causes a substantial environmental load.
It is a purpose of the invention to provide an X-ray tube with effective and compact shielding against the undesired secondary radiation, said shielding having minimal effect on the dimensions of the rotary anode assembly. This is achieved in the X-ray tube according to the invention, which is characterized in that the shielding means comprise a substantially flat shielding plate within the tube housing, which shielding plate extends transversely to said rotation axis and is positioned between the cathode and the emitting surface of the target disk. The efficiency of the shielding according to the invention is explained by the fact that the secondary radiation originates substantially from the emitting surface of the target disk of the rotary anode and is intercepted by the shielding plate in the direct vicinity of its source. Due to the fact that the X-rays are emitted in the 2xcfx80 solid angle, substantially orthogonally to the incoming electron beam, the emitting surface of the anode is oblique relative to said beam. It is possible, therefore, to introduce an inner and an outer limit of the emitting surface. The useful X-ray beam will constitute only a part of this 2xcfx80 solid angle. The placing of the shielding means according to the invention could be selected in such a manner that the shielding means maximally approach both the emitting surface of the rotary anode and the electron beam from the exterior of the rotary anode assembly. The effective solid angle of the shielding means will be, therefore, optimized. A further embodiment of the X-ray tube according to the invention is characterized in that the shielding means further comprise a ring-shaped projection on the surface of the target disk facing the cathode. This arrangement of the rotary anode will shield the ambient space from the electrons which had undergone a scattering on the emitting surface of the rotary anode together with X-rays which are produced by said electrons. This undesired secondary radiation is emanating in the direction towards the rotating axis of the rotary anode and will be intercepted by the ring-shaped projection on the surface of the rotary anode. A further embodiment of the X-ray tube according to the invention is characterized in that the shielding plate is ring-shaped. This arrangement of the shielding means is conform to the shape of the target disk of the rotary anode. The useful part of the X-ray beam will be transmitted through the tunnel formed between the emitting surface of the target disk of the rotary anode and the shielding plate. It might be advantageous to select the outer diameter of the shielding plate the same as the outer diameter of the target disk of the rotary anode. This arrangement will not enlarge the outer size of the rotary anode assembly and will contribute to the minimization of the X-ray tube dimensions. A further embodiment of the X-ray tube according to the invention is characterized in that the shielding plate is fixed to the rotary anode with fixing means. In this case the target disk of the rotary anode constitutes a bearer of the shielding means and no additional mechanical construction is required to support the shielding means. A further embodiment of the X-ray tube according to the invention is characterized in that the fixing means comprise cylinder-shaped pins attached to the surface of the shielding plate, said pins cooperating with holes in the target disk. Another embodiment of the X-ray tube according to the invention is characterized in that the fixing means comprise rigid projections manufactured on the shielding plate said projections cooperating with notches on the target disk. In both given embodiments of the fixing means it is sufficient to select three fixing positions on the surface of the target disk of the rotary anode, said points being separated from each other by about 120 degrees. In some situations, where the temperature of the target disk of the rotary anode induces the problem for the mechanical stability of the fixing means, one might add a temperature barrier on the back surface of the target disk. This temperature barrier can be implemented by a thermally conductive element with a limited cross-section, said element connecting the back surface of the target disk with each fixing element, respectively.
It might be advantageous to assemble the shielding means to another mechanical bearer than target disk of the rotary anode. A further embodiment of the X-ray tube according to the invention is characterized in that the shielding plate is fixed to the cathode. This embodiment uses the fact that the cathode is stationary with respect to the electron beam and thus to the source of undesired secondary radiation. By fixing the shielding means to the stationary structure, like the cathode, one can further minimize the dimensions of the shielding plate, as the undesired secondary radiation emanates in the limited solid angle. A further embodiment of the X-ray tube according to the invention is characterized in an inner limit of the shielding plate extends to the rotation axis to a distance smaller then a distance between said rotation axis and an inner limit of the emitting surface of the target disk. This construction of the shielding plate effectively shields the ambient space from the scattered electrons and X-rays which are emanating in the direction towards the rotation axis of the rotary anode. This shielding plate can also contain an exterior part which projects towards the outer limit of the target disk of the rotary anode. Evidently, there must be an opening in such shielding plate to transmit the primary electron beam. This exterior part will create a tunnel for the useful part of the X-ray beam in a manner, similar to that of the ring-shaped shielding plate. An advantage of the shielding means arranged in this way, is that the dimensions of the shielding plate can be minimized to substantially cover only the solid angle of the secondary radiation and no further fabrication steps for the rotary anode are required.